Apparatus foe rectification of air



ocr. 23,1923. A 1,471,333

H. N. DAVIS APPARATUS FOR RECTIFIGATION OF AIR Original Filed July '7, 1920 4 Sheets-Sheet 1 wam/s N2 our mu/o a, our

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Oct. 23 1923. 1,471,833

H.rq.:mAv|s APPARATUS FOR RECTIFICATION OF AIR original Filed July 7. 1920 4 sheets-sheet 2 -J Elwin/wkn 4f @my/Mms w @www Amm www Oct. 23 1923. 1,471,833

H. N. DAVIS APPARATUS FOR RECTIFICATION OF AIR Original Filed July '7. 1920 4 Sheets-Sheet 3 Oct. 23, 1923. 1,471,833

H. N. DAVIS APPARATUS FOR RECTIFICATION OF AIR original Filed July 7. 1920 4 sheets-sheet 4 551,3 @mwa 7W Patented Oct. 23, 1923.

UNITED STATES IA'ZLSE PATENT OFFCE.

HARVEY N. DAVIS, OF. CAMBRIDGE, IvIA-SSACHUSETES, .-.SSIGNOR TO RESEAIi/CH COT.-

IORATION, 0F NE?! YGEI'I, N. Y., COItPORA'IIQN NET AFPAREUS EOR RECIIFICATIGN DI? AIR.

Original application led July 7, 1920, Serial No. 394,544. Divided and this application filed November To @JZ ywhom t may concern.'

Be it known that I, HARVEY N. DAVIS, a citizen of the United States, residing at Cambridge, in the county ot Middlesex and State ot Massachusetts, have .invented certain new and usetul Improvements in Apparatus for Iiectiiication of Air, of which the 'tollowing is a speciiication.

The present invention relates to air liqueiiaction and distillation processes andto apparatus such as is novv commoniy used for the separation of pure oxygen or nitrogen, or mixtures rich in oxygen 0r nitrogen, from the atmosphere, and more particularly to those parts ol such apparatus which are commonly called columns or stills. The object of the invention is to make apparatus of the said kind more eiiicient, to reduce the power required to effect the separation ot a given amount otusetul product, and to decrease the cost of the useful product, which may be either oxygen or nitrogen oic any useful degree of purity, or both.

It should be understood that my inven tion, at least in some of its aspects, is applicable to apparatus tor the separation of mixtures other than air, which liquety at loiv temperatures, and that, in describing my invention with particular reference to the separation of air, I am not limiting myself to reference to this particular application of it.

For a full understanding of the invention reterence is had to the accompanyingdrawings, in ivhich Figs. l and 2 are diagrams introduced solely for eipository purposes.

Figs. 3, el and o are diagrams representing various alternative arrangements ot rectification apparatus or ot parts thereof embodying the various elements ot the invention in alternative forms.

Before describing speciiic embodiments of the invention, I desire to explain certain underlying fundamental principles, an understanding oit which will maire the details of the various alternative embodiments much easier to tolloiv. There are tour ifundan'iental principles that have guided the design of all the alternative arrangements proposed. These four principles are I. The main rectification train should be run substantially at atmospheric pressure,

Serial No. 424,760.

and should be ted with as much unliqueiied and substantially uncompressed air as possible.

II. Adequate means should be provided tor transferring heat to or from the con tents ot the main rectification train at all levels, or, it this be impracticable, at a plurality of ivell distributed levels, and not merely, as is substantially the case in the present state ot the art, at the top and bottom.

III. Appropriate streams ot i'luid of different compositions should be ted into the main rectitication train at as many different, Well distributed levels as possible, so as to minimize the heat transfer requirements mentioned in II.

IV. There are certain levels in a rectilication train designed in accordance With I, II, and III at which longitudinal heat leakage is especially detrimental, While at other levels it is much less detrimental, and may even be desirable to a certain extent. rihe structure containing the rectification train, may, therefore, advantageously be divided into tivo or more parts, connected by relatively slender metal parts, in such a Way as to minimize longitudinal heat leakage at those particular levels Where it is most detrimental.

These tour principles may be further eX- plained as follows:

I. The rectification train should be run substantially at atmospheric pressure and should be fed with as much unliqueied and substantially uncompressed air as possible.

I am Well aware that there are certain advantages in running the main rectification train under a `much higher pressure, in that it is not then necessary to maintain temperatures as low those which must exist in a loiv pressure rectiiication train, and in that the greater densities of the gaseous fluids handled means a more compact column for a given output, both oi" which considerations indicates that a high pressure rectification train vvill have smaller heat-leak losses than a lovv pressure train.

@n the other hand, ruhr-.ingv the main rec tiiicaiicn train initier pressure justified from the power standpoint only when a considerable part ot the Work required :tor compression recovered in some usetul (lll form, as in at least one system previously patented. But unfortunately the etliciency ot compressors and expansion engines is such that the increased losses in these inachines will much more than overbalance the increased losses due to heat-leak in a low pressure system. The essential teature ot such a system is that as large a part as possible ot the air to be rectified shall be ted not only into the main rectification train itselt, but also into the system as a Whole, at the loivest possible pressure, that is to say at a pressure above atmospheric by just enough to keep this air in motion through the system against atmospheric pressure at the various outlets, and to admit ot means tor controlling` this tlojv. lliccording to my invention the only air that is materially compressed at all is that amount Which will just sutlice, under the most effective arrangements that can be devised, to furnish the heat-pumping power required to keep the system in operation.

ll. idequate means should be provided tor transferring heat to or from the contents oi the main rectication train at all levels, or at a plurality ot Well distributed levels and not merely, as is substantially the case in the present state ot the art, at the top or bottom or both.

lllhis proposition, which is oit great importance, can best be explained by reference to Figs. l and 2. Fig. l represents diagramniatically a rectification train et the simplest possible type. Unliqueed air enters the train through the pipe l., gaseous nitrogen, ot any desired purity, leaves the top oit the train through the pipe 2, and liquid oxygen ot any desired purity leaves the train through the pipe 3. The interior of the shell may be illed With a large number ot trays ot any Well knoivn type, or vith battles, glass beads, hollow aluminum balls, or any other Well known structures, ivhereby stream ot gas rising through the shell under a slight pressure gradient, is brought into intimate thermal and physical. contact vvith a stream of liquid descending through the shell under the action ot gravity. rlhat space in a column in which an ascending stream ott and a descending stream oit liquid intermingle and interacV together With the structure containing and con rolling the streams Will be reterred to throughout the specification and the cle-'fins rectilication train, as distinguished trointhe column itself Which may and usually does contain, aside from the rectification train or trains, other structural elements or groups elements participatin the process ot rectiiication. it is eed that at every level the compositions t t gaseous and liquid streams are such as to be in equilibrium with each other in accordance with the well knoivn laws of two-phase binary mixtures, and the experimental tacts represented by the Well known Baly curves and other similar experimental data. Wie assume further that the temperature at the various levels in the train varies uniformly with height trom that appropriate to orthobaric, nearly pure oxygen liquid at substantially atmospheric pressure, at the bottom ot the train, to that appropriate to saturated, nearly pure nitrogen vapor at substantially atmospheric pressure, at the top. rind finally ive assume that the air inlet pipe l is that level at which the rising gaseous stream in the train has the composition of air.

Vite then enquire under What conditions this rectilication train could theoretically be operative in the manner assumed, Let us consider any level PQ. in the lower part ot the train, at which the temperature is 'l`. rlhere will be a stream oft liquid tloiving down past this level containing, say, m niels oi oxygen and fn mois ot nitrogen. There will also be a stream ot gas flowing up past this level, containing, say, M mols ot oxygen and N niels ot nitrogen. To determine these tour quantities, fm, n, lill and hl as functions ot the temperature We have the tour conditions; lst, that the ratio ot m to n must be such as to give the liquid composition corresponding to on an oxygen-nitrogen tivo-phase equilibrium diagram such Balys curve; 2nd, that the ratio ot lvl to ll must be such as to give the gaseous coniposition corresponoing` to rll on an oxygennitrogen tivo-phase equilibrium diagrziun such Balys curve; 3rd, that (m M) must be equalto the quantity ot oxygen leaving the train at the bottoni; and 4th, that (1w-N) must be equal to the quantity ot nitrogen leaving' the train at the bottoni. lit the tloiv and composition el' the desired oxygen-rich product are assumed, these tour relations completely determine fm, u, and N as functions ot T liroughout the l iver part ot the train. Similar considerations determine them in the other parts of the train. rthe results ot a typical case are plotted approximately to scale 2. ln this case. the quantity oi liqi'i be 'flowing dovn 1east each level must vary 'troni ,Level to level in :i ly the manner shown by the grot. i, 2, in Which horizontal distance inols and vertical distances rc temperatures or heights rectiiication train ot l, and in uni/ln as is common in theoretical discussie the ro ti Vhcation is assumed to he t ihuted coutinuously over the height oi f c in. rilhis graph a ot 2 shoivs that in a theoretically perfect rectification train oit the type described, the quantiy ot matter in the descending liquid stre roust he Zero at the top of' the train and must increase,

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at first very rapidly7 and then more slowly, until the stream reaches the air inlet level l of Fig. fl, From this level down, the quantity oi matter in the descending; liquid stream must decrease, at first with some rapidity, then more slowly, and finally with considerable rapidity to value at the bottom of the train that represents the quantity oi nearly pure liquid oxygen leaving the train through the pipe 3 ot Fig. l. llroin this it is evident that there must be con- @lensing action all through thC upper part of the train down to the level l, and vaporizingz; action all through the part ol the train llelow thl level l. rllhis means that heat must be itlidrawnfroin the contents ain :rectification train at all levels ot the above the level l and that heat must be iniparteu Ato the contents oi the main rectilicat-ion train at all levels helow the level l., il' tho train is to function in the ideal manner a: 'tuned above, heat that must be transferred either to or troni the contents oit the train at the various levels is indicated approiinately by the graph ji of 29 in which areas to the right or lelt oi the vertical reference line represent heat units to be transferred within the corresponding' temperature intervals in the train. Y

'it is therefore obvious that it a train of the type of l is to be made to operate in the assumed manner, means nist be provided for ell'ecting Athe necessarj7 heat transvters at the various levels. This is best done hv auxiliary streams or" fluid which it is desirahlm or at least therniocl'vnai'nicalliY permissihle, to warm or cool as the case may be. Two such streams ot luitl are obviously available7 even in the simple train repre sented in F l. n

une oi these is the stream ot cold nitrogen product from the top of the train, which is, in the present state of the art usuallv carried direct to the much warmer interchangingg system. This stream should he car d down through coils or oth' passages in intimate thermal but not phvsical contact with the contents of the rect'cication train, at least as far as the level l ot Fig. l, so that its heat absorbing power, which available through the coldest and therefore most unnianageahle tenr perature range i i the whole apparatus may be made use ci in the most eilective possible wav7 nau'ielv in helping solve the heatremoval problem inherent in the upper part oi" the main rectification tr Furthermore the air enter J4 '1s through the pipe iup;V the train leiit the main heat interchangmu system f. a temperature substantiallvequal to that appropriate to the bottom ot the main rectiiication traiin and its power to ,Qive up heat as it cools from this temperature to that ap priate to the level l7 is available throu hout the tern-` l has rllie amount olV he passed through coils or other passages iu u ciale thermal but not physical con tact with the contents ot the main rectilication train through the whole or a conf-iiderahle part ol' that portion oit the train below the level'at which it enters the train.

Several other advantageous means ot transferring heat to the contents oi? the lower part et the main rectification train will he rilescrihecl in later parts oi these speciiications, including streams oi high pressure air which it desirable to cool, streams oi intermediate pressure air which it is desirable to cool and partly or wholly condense7 and streams oi nitroggen which it may he desirable either to condense or :rnereli7 to cool. Each ot these means will he mentioned in connection with another specific feature to which it is appropriate, but it should be understood that each has also the novel and great advantage of play* ing its appropriate part in elicctiug the necessary transfer ot heat into the lower part oit the Ina-in rectification train.

Cert in advantageous means will also he described in detail later for taking heat out of the upper part of the main rectification train hv means ot streams of fluid in thermal but not physical contact with the contents ot the main train various levels thereof. ln particular is soniietimes advantageous to torni substantially pure nitrogen liquid in a manner to he described laten and to pass a part oi this liqul through coils or other passages and evap* orate it in thermal lout not phv cal contact with the contents oi' the main rectification train at various levels between the level VA l and the top of the train. It is also sometiines aclara-iro'ageous9 as will appear later7 to form other liquids richer in nitrogen than the liquid in the main rectification train appropriate to the level l in Fie: 'l and to pass parts ci these liquids through coils or other passages and partially evaporate thein in thermal but not physical Contact with those portions ot the main rectiiication train between tie level l and the levels at whichr the partiallgr evaporated liquiils may propriatcly he mingled with the streams the main train.

llach oi these procedures has great, vantaaes over putting' the streams of liquid oirectly into'the main train at level helow those to which their compositions are appropriate. Each has also certain advantages, in seine casesq over puttino; the whole of each stream into the train its appropriate level for reasons thatiwill appear later. These procedures aro mentioned here as eX- ainples oli' ways in which heat can loe abstracted `from the upper part of the main rectiiication train by cold streams of fluid other than the stream or nitrogen product mentioned above.

lll. The type oit rectiiication train shown diagrainniatically in Fig'. l could doubtless be made operative by tl e :'neans described above7 but the amount oia necessary heat transiter is large and7 particularly in the upper part of the trai", difficult to effect in a practical manner. lt is therefore advantageous to niodify the rectification train itself by introducingl streains ot iiuid otiier than the single stream of gaseous air represented in Fig. l as ente-:ingry through the pipe l. ln such modifications it is a fundamental thermodynamic principle that each streain should enter the inain train only at that particular level to which its composition is appropriate. To secure the best results in niininiisin?` the necessary heat transfer for ideal operation, or, in other o/'ordsu to so modify ideal heat transfer curve A of Fig. 2, it shall lic as nearly along its vertical base line as possible it is desirable that such streams as enter the train above the low pressure air inlet level l of l shall be Wholly or largely liquid, and that such streams as enter the train below toe loiv pressure air inlet level shall be wholly or largely gaseous. Means for providing` gaseous or largely gaseous streams ot coin-position lor entrance below tl'. are described in a copendi applicati Davis and l liled AJuly T, 1920. Means 'for providing liquid or largely liquid streams oit appropriate composition entrance above the level l will noiv be described.

The niost important liquid or largely liquid streain that can be ted into the upper part oit a rectiiicaticn trai-, is a stream or pure or nearly pure liquid nitrogen at the top of the train. Such a streani is already coininonly used in the art. It is Yfoi-fined by sonic (see for example U. S. Patents No. l,20l,01ll and No. 11.252470) by supplying under pressure a stream ot previously puriiied nitrogen, which cooled in an inten changer, condensed by thermal interchange `with a pool of the liquid oxygen discharged from the bottom of the rectification train. and then throttled in to the uppermost part ot the train. This procedure may however be 'improved in tivo Ways. ln the first place the pool ot liquid oxygen led from the bottoni oi the train is not the best available place to condense this nitrogen, as this is the -warmest part ot the coluinn or still, and the pressure under Which the nitrogen stream is supplied inust therefore be higher ir" it is to be condensed in this Way, than would be necessary if condensation is eiieeted in other and cooler parts ot the train. A better Way oie condensingl a stream of nitrogen for use lleinschniidt Serial llo.

passages in thermal but not ph tact with the contents o'f that i of inain rectiiication train that lies just belou7 the loiv pressure air inlet level l. oit Tio' 1l the pressure oi2 the nitrogen b just uiuoient to cause condensation under -liese niuch niore favorable conditions. ylliis procedure has also the advan 'Mge ot Kowing heat to the 1 r .Il ..1 A a .-f L cont-elias ci the inain iectiucation train in a OU-m i .4 n .11. la a Af1 *1' retries Wncic, aecoi oM o grap i il oi rig. 9 the desirable heat l i sier rate nieteiially larger than somewhat lo ,er levels Furthermore, under these conditions the nitrogen st-reani can be cooled in coils or other passages in thcrinal but not physical contact `with the contents ot the Whole lower 2L. t of' the train, ivith the double advantage ot promoting the condensation or" the nitrogciu and supplying heat to the contents ot the niain l at all levels belowv he level l of l, in the inanner previously mentioned.

il ith seine dispositions or the rest ot the apparatus, the ani or" nitrogen nientioned in the last paragraph niay be iinoiily as supplying Waslrliquid tor the n oerinost part ot the ieetiiication i but also because ot its potential heat newer. Under these coniglitions it cti is desirable to pass it to the coliinin under a migrer pressure at centen ted in i agir-apli. li', hor-sover e presot this streani is raised only inoderately above that necessary to secure condensation before the streani leaves the loiver part oit the coluinn, either condensation with lat heat absorption7 or abnormally liio'li si heats Will appear the niain intel-chauY system, with the disacvantag'cous results de scribed in iny Patent No. l f 20.625 lgranted June 27, 1922. Under these coi oitions. the said streani of nitrogen may Well pass through the interchanging system and enter the coluinn or still.. at or its loiver end, under a pressure that is materially above the critical pressure oit nitrogen, and pass up through the Whole lou part ol the rectification train coils or other passages in tlierinal but not physical Contact with the contents oie the train, thereby giving;r heat to the contents oit the train all levels belouT the level l ot l and loing itself cooled substantially to the temperature appropriate to the level l. Without any condensation Whatever. lllhen this stream olf' high pressure nitrogen is throttled fron the level l into the top o the niain train. liquid Will be it'ornied abundantly.

i second method ot providing; the nit-rogen required to forni the sail liquid stream., also Well known in the art (see 'for example llt) U. S. Patent No. 981,748) consists in so treating a stream ot air under pressure as to leave :i gaseous residue which is nearly pure nitrolnen, and subsequently condensing this nitrogen substantially in the manner described above. This procedure has a. considerable advantage in that a separate nitrogen compressor' and a separate supply of previously pur'lied nitrogen are not necessary. This procedure, as now lniown to the art, can, however., be much improved by the application of the 'fundamental principles already described in these specifications. For example` since the process of condensation and rectification that goes on in the ascending or backward return tubes in the lower part of the well lruown Claude column, requires the removal of a considerable quantity of heat at all levels, and since the process that goes on in the lower part of the ideal rectification train oil lFig. 1 requires the absorption of heat at all levels, it is obvious that each process will be furthered by putting it in thermal but not physical contact with the other. This means that the ascending or baclfward return tubes of the well known Claude column should be in thermal contact not merely with a single boiling off' pool at the bottom of' the column as .in U. S. Patent No. 981.748, or even with two or three such boiling ofi' pools as in U. S. Patent No. 1,083,988, but with the contents of the whole lower part of the main rectification train from the bottom thereof up to the lowest level at which any material to be rectified enters the train; that is to say, the (llaude backward return tubes should be surrounded by, instead of merely surmounted by, the trays of the lower portion of the main rectification train, in a manner to be described later in connection with Fig. 5.

lt is also obvious that, because of the shape of the ideal heat transfer graph A of Fig. 2. and in `particular because ot the fact that the amount of heat. that should be given to the contents of the main rectilication train at levels just below the low pressure air inlet level greater than at somewhat lower levels. it will be advantageous in some dispositions of apparatus, for example that shown in Fin'. to make the nitrogen condensing tubes much shorter than the bachward return tubes` or to replace them by coils or other passages surrounding onliT the upper part of the baclvvard return tubes. so that their heat yielding power may be concentrated in a part of the main rectification train where it can be used to the best advantage.

Other means for termino from air under treatment the nitrogen desired for the upper part of the column will be described in .later paragraphs of these specifications.

The effect of a stream of liquid nitrogen, :introduced at the top of the main rectification train, on the ideal heat transfer graph for the upper part of the train, is shown by graphs o and B of Fig. The introduction of a certain amount of nearly pure nitrogen liquid will just suffice to reduce to'zero (or preferably to the magnitude of the inevitable heat leal: into this part of the apparatus) the amount of heat transfer required to maintain the uppermost partei the ideal rectification train of Fig. 1 in operation, but the effect on the ideal heat transfer requirements at levels just above the low pressure air inlet is small.

If a considerably larger quantity of nearly pure liquid nitrogen is introduced, conditions are modified as in graphs c and C of Fig. 2. A considerable quantity of heat should new be put into the uppermost levels of the train, and it' this heat is not supplied by special means or by heat-leak, the whole uppermost part of the train will cool toward a uniform temperature, and the rectifica-` tion effect will be crowded down toward the low pressure air inlet until the longitudinal heat-leak in that part of the column is sufficiently increased to provide the necessary heat. That is, the uppermost part of the train, will be drowned with liquid. lf, therefore, an excess of nitrogen liquid is available, it is not desirable to put all of it inte the top of the train as is now commonly done in the art. It is much more advantc geous to throttle a part or all of it into coils or other passages scattered through the whole upper portion 01"' the main rectification train, and particularly in regions thereof just above levels at which streams enter the train, as in Fig, 4, so that a part of the said nitrogen liquid may be evaporated by heat withdrawn from the contents of the main train at these levels, and so that the quantity of liquid actually entering the uppermost part of the main rectification train may be not greatly in excess of the amount needed to make the desirable heat transfer rate there equal to the inevitable heat-leak rate.

If other liquids. richer in nitrogen than the liquid appropriate to the level 1 of Fig. 1, are available, and are introduced in appropriate quantities, each at the level to which its composition is appropriate, the effect on the ideal heat transfer curve of the upper part oi the train is very marked. Each stream reduces the desirable heat withdrawal rate at the levels immediately below its point of entry, and may maire an input of heat desirable just below its point of entry. Indeed if a very great number of such streams ot appropriate magnitudes and compositions could be provided, the ideal heat transfer curve could be made to substantially coincide with its vertical baseline, or preferably with the line representing the inevitable heat-leak distribution.

.veil under practical condi'lliors several all liquid streams 'll he sort under consido eration, by Luo-,seing such Siti-eamo can be fed .into the Lo- L them Jahrough coils or per par of a rectification train oli? V"he other paese, l iii thermal but hol; physical type herein described Willi ad" conlacfo il i he coiieiiss of the main rectivanlooe y. i Well aware of i'he lach iieaioii Jrain ifouksrhoat the regio-o below wat i a plurality suolA lvhe lowes@ leve :ifo v.' lich material enters the Streafiis been lcecribed by oehio 1r, e ppl l stream o 'e1-ing lhe upper ha'f lion rain, al-

llig". n helling il c l'elthe a l for edvanliageou.

l have furlheriiiore,

meilbfle ol forming; one or more Fiese tion of apparatus and che eco? S'lzreams o'her their lhe ubporroie or io i oilmy invention Shoull not l pure nirogen stream vioueljT dl i to be l' ited to those partici iii particular one auch which 1 have found it coiiveiiioir; to Show ii` easy c a ,alream opanyiiig draaviiigs,

Sure is Woolly iioieiieo, en leasI iheorelzicaiiy dealrable lo cess described by Claude in ,arabplv to lhe upoe: part o'l the m lo rodol- 967,104; and elsevli sages ii a pool of liquid obm'eii end of l" A corri iicarioo train as man v largely liqui l e ent romposiiorie possible, and coiiiposilmii adoi'iooal etreariio 0'? Suitable can be formed in o variegv or. Wars oarticular, the fairiiliar Claude backward rehave lhe coinpoeif" liar; tubes, either iii their usual form, or in it has been lrotl'- Y the woei red disbositioii shown in Fig, 5, can mam trail, wi h Some l '4l ed with advantage, bv

l 7 ing evapora iai, rhein into wo more be much richer in hir g p rposed portions, with meal. c l `ing various levelis the liouil ooicil the jorliona or the lobes in immediaelv above each said level, a @fl mea is for pasarme these liquide, Vwhich will ,i Similar arrangement is d .i Claude@ cloned above. ll. Si. ,9 S, but the migo Paeir No. 1,083 y lJY Y U10/il" here proposed h two conside:fehle ment rlhe process ioned above, and other stre?. hs herein gdvmh'apeg- 1n "che li place. since the weiiioiied aS eht-Grioo* die main reeiiieaioii mbeg ghlmll, 3S has been mniimred, extend lraiii at levels above the low presume air up through fh@ who@ 10W@ mi 05th@ TQQ- ll lfel l l, 'fmlfll may ticatioiibtrain, much cooler temporatireo vaiitageously be modified follows, 1'@ io we amigable fm- -he 195591- 10a-,LS of @is TGC?. Obvio that il: SUCli SCVORWS @U2 b3 `l v ticaiioii lhaii in his diepoeitioii oil ab a Cooled af "r lili; Wl I l raus, arid so ibo pressure ineide the Uhr under or Ward return tubco can be som-Lavha reduc e. 11nd iii the second place` portioiir; ol' 'l .tubes io iov i u'm' aficiones flzzreaofrrw, @o rofflil'rzr elnvard refaire is efliceiire in all o instead ol merel in ectionfs of the tube eysllem, as in y iomric ier in nitrogen. Means `or elle further cooling' iii the reale loro" have already been dosiscribcd by oll ihparrieular bv Claude in ll. S, L 961,10?. But iiiasiimch as ino c i ,reii'lj/ been aiva "e hitherto o J@he desir b'litjv of imparliiig; heat to the conl'eiits the main reci-L "fori train at all level i at which diete` ia the train. no one o s g, seen the desirabihby oz still Afurther aowv *Jorn o lie last of damental principles meiiioied in pari; of these specifications. A reel* raiii designed in accordance With r ciples already discussed will be 'fed by two lllO lll

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or more streams of inaterial, at least one of which will enter the train at a iioiut intermediate between its highest and its lowest level. ',lhe rectification train may therefore be r ,ggarded divided bythese levels at whi .finaterial enters it, into two or more portions, cach of which will hereinafter be called. a run of the train, each run being a portion ot the train within which there is uo level at which material enters the train.

li the amounts of' material entering the train in each stream are wisely adjusted, each run except the lowest will have an ideal heat transfer curve such that heat should be supplied to the contents of the ruu at all higher levels thereof, and heat should be abstracted `trom all the lower levels there of. in trie lowest run heat should be sup* plied at al levels, no matter how the streams are adjusted. lt follows that all regions just above, or rolder than, a level at which material enters, should be losing heat, and all levels just bel w, or warmer than each such level should be receiving; heat. But the tendency ot the inevitable longitudinal heatn leakage iu the train is, iii the immediate ueihhorhood ott each such level, exactly opposed to the ideal state of allairs just men`- tioned. The upper hart of each ruu, which should be gaining heat, will lose heat by longitudinal transfer to the lower part et' the next higher run, which in its turn will ,gain this heat, when properly it should be losing heat. It is therefore precisely at those levels at which material enters a rectification train` that longitudinal hoat-leal is .roost undesiralile. I there-#tore propose as a new and. useful improvement the design and construction of a rectification train, 'that it he divided structurally into two or more scharato portions al; one or more of the levels at which material enters it, that those portions be insulated from each other as woll as 'troia their surrouudino's in the most effective practicable way, and. that such pipes aud other connectious between successive portions of a rectilication train as may be necessary, he so arranged to provide as small an opportunity for the leal@ aire oli heat as may he practicable. For the saire ole simplicity l have shown only one such divisionA` the principle heini! 'thc same irriviiwctivc o't the number oi divisions. rlhis :livisiiui ot the main .rc .ti-cation traiu iulo two or more structurally distinct uur. tioiu; iuav also have certain other advantages, which will be uieutioued later iu couucctiou with Fig'. ll.

lllithiu any one oi the upper runs o' a rertitication trahi, the eil'cct ot longitudinal heatrlealr is quite di'tlerent The lower or warmer levels oit such run should ideally lose heat, and the upper or cooler levels should ideally receive heat, and this is ore-Y cisely what longitudinal. heat-leakage af'- coinplishes. No unusual precautions are 'therefore necessary to prevent longitudinal heat-leak within any one run oL- a rectiica tion train, and it is even `conceivable that under soine circumstances it inipj'ht be desirable to encourage longitudinal heat-leak withinv a siugle run by the introduction oi.t copper rods or the like, if other and better means lor securing the desired transfers ot' heat are for sonie reason impracticable.

A much'better way ot ensuriup; appro priate heat transiter in any one el? the uu Each such run should, has been. tioned, receive the Ltull benefit of l' co ing or heat absorbing -cov-.ier et the stream of nitrogen product from the top ot ile still, over the temperature rauge correspond inf; to the run in question.` .l3nt in a thermally well balanced run only the lower.' levels need to lose heat. Therefore it is advisable to bypass the nitrogen product stream past the upper levels oit the ruga and to concentrate its whole heat absorbing power, over the temperature range in question, into the lower levels oithe ruu from which heat should be abstracted Furthermore the various streams ot liouitL that are to be throttled into the upper art of the raain rectification traiu have beau do scribed as passing, while still under presH sure through coils Aor otlrr assao'cs in thermal but not physical coutat i. ,h the conteuts of the main train thr gh the whole length ol the lowest run thereofu lt now appears that each of these streams :hav also be passer, while still under pressure, through coils or other passages in thermal but physical Contact with the uriner levels ot each run uu to the level at which each said stream to outer the main train, with advantage both to the liquid streams tliemselves, which 'it is desirable to cool as rauch as possible he'tore thcv are throttledi, and to the a rectilicatiou traiiu to which. it is desirable to im yart lar levels in question.

Having' how described. the fundamental principles which underlie niv heat at the partie system or process for seriarature)v air or other gaseous mixtures luto tho coustitueuts thereof. and

having meutioued man" ot the oovel aad useful features there l will describe briefly as possible three alternative euib ments oi" some or all ol the features o?" niv u veutiou and explain the operation thereof, poiuting;` out in coanectiou with each. such riovel features particularly iu construction or inode of operation, as have not already been mentioned.

Fig. 3 represents diagrainniatically an in terchanger ll and a main rectihcation train contained in a shell or column 12.

The im eil terchanger which is more psrticulerly described in my Patent 1,420,625 carries three doivugoing streenis 18, le and l5 composed, respectively, ot L. l). air, Tbl. air und P. nitrogen, and two tip-goino' streains, l@ and 17 oi products troin the eoiuinn or still. l't muy be statedlie e that. pressures not nieterially higher thun atmospheric pressure will be throughout reif to as low pressures (L. l), pressures Well above the ciiticul pressures ot the iluids iii question es high pressures (ll. l), and pressures i iatcrlslly higher than atmospheric pressure und belen7 the critical pressure as interine-diete pressures ylhree expansion engines 1S, l@ a ted by l). fluids, are provided in accortience with the principles described` in said application. ln this particule-.r case t coldest engine 20 is a nitrogen ei and exhaust l. l). nitrogenv which is used in the still after the Well known niethoL of` ard-lllainand- The other two engines l and i9 receive lil. gir one duct "iltlie interchs-.ngcr into ivhich it is rorced by e.. suitable compressor or compressors und exhaust` L. l). air into another duct indicated.

rlhe main rectification train in the column 12 consists olf thr e runs deined by the level of the inlet 28 through which li. l?. gaseous air is introduced into the columnq und by the level of the inlet lll et which lil. air is throttlecl into the coluinn utter beine' so cooled as to liquety alniost completely the throttle 44;.

Each ot these streams is put in therinel Contact With the contents ot the loiver run up to the level ot the low-pressure air inlet 23 by beingi passed through the coils 30 4:0, respectively. Y/dtlitional heating;` power ilor the lower run and additional cool power for the lower portions of the two other runs are provided by utilization oit the lil. nitrogen stream.

Freni the interclianner ll part of the l). nitrogen stream l5 is carrierL h v pipe t'o the coils i3 in the lower run. This stream is then, in this particular instance, divided and throttied through the valves il@ and i9 into the coils il@ und t7 respectively in the lower portions ol: the two upper runs. ll'ie rci'ueininf; portion, it any, oi" this nitrogen streaui is throttled through the valve into the top oit the main train.

The nitrogen product 'troni the top ot the coluiiin or still passes through the pipe 24.- into and through the coils Q5 and QG to th bottoni et the rectification train, hein?,` there by warmed substantially to the temperature appropriate to the bottoni of the interd cha-nger to Which it passes through the pipe 17', and being; thereby caused to yield to contents et the main train ell ot its available cooling power, which is thus used iior the purposes described above, and to assist in keeping; the trein cold in the 'fece oi the veiious ine-ital e hout-leal rllie ygen liquid leaving the rnain rectilication train the b ttein thereo'l' is deiiected by the di pliregafi 3l into the 'veli 32 e it coiuef ite tl'ierinal e ntact with a oil' the l. nitrogen the coil 33 a sul'iiected to e s `ilcation by i -Hliintensity ci which is regulated liand valves Eli'. ihre liquid ss/s through the trop inte fp. 'L ou pool e5 Where it is vey heat exchange with the l. cov neeted the coils which are by pipe 20", with the exhaust side ot the enrThe c product then leaves p i6. in the ineann he lnrs'elv or uholl r liouced l).

nitrogen treni the coils 38 and is throttleo. into the top oit the main train through the valves 37, the riser end Jthe valve 39.

Starting); up such en apparatus as that rseiitel diapjraininatically in 3 inirgit resent certain dill'icultics with rcspeet to the supply of r :ogen Jfor the high pressure stream l5 during' the period while the eupsratis i cooling; down and beginning il. I 25 end 26, etc. und finslly the increiA il. fis the system cools down,

this air will he liqueietl by the again rear-hes the space at the top ot' Furthermore, the ascending colot r1-as or vapor in the still ivill begin to drop seine liquid before it reaches the space 23. Since both ot these liquefied. portions Will he richer in the less volatile constituenthere oryggen\T then thestreains troni which they are formed, thI remaining gaseous streains reaching; the space Q3 will he richer in the inore Volutile constituent (here nitrogen) than when they passed their respective compressors. ie streani l5, which is continually 'lied the sueco will thus grrovv richer in the more volatile constituent (hee nitrogen). lt is possible that this progressive purification oit the contents ot the space 23, `troni Which stream l5 is ted, Will proceed rapidly enough to keep pace With the settling` or the rest of the apparatus into its steady state.

lf this proves not to be the case, I propose two alternatives, each ol` which involves only no lerate storico facilities `for pure nitro Y accumula 1 in a previous run. The nrst depends on the tact Athat the slow part oi this progressive purilication will be the last part, atter the contents ot the space 23 arenearly pure, but not quite pure enough to give the best results olf which the apparatiis is capable. l then propose to shift teml'ioi'arily the source oit the stream l5 from the space 23 (via the interchanger) to the stored supply ot pure nitrogen. This will supply to the space through the valve 39 an abundant supply ot pure nitrogen liquid, which will gradually wash oxygen out of the ascending column of gas in the still, and iiorce the upper part of the still into the desired steady state. This requires a storage oi nitrogen only suthcient tcsupply the stream l5 while the out-going strean'i 1.7 is settling into its iinal pure or nearly pure state.

.lit is possible that even this accelerated process of progressive purification of the stream l? will be too slow from the point oi: view either ot convenience or `oi expense.

li" this proves to be the case, I propose the lollowing, more far-reaching alternative, indicated in `Fig. 4. Here the still or column is separated into two parts at a point corresponding to the level of inlet 2S in Fig. 3, and means are provided whereby the ascending column ot gas or vapor in the lower part of the still may be diverted at the level oi? inlet Q8 and returned through the pipe to the 11p-going stream 16 in the interchanger system, without traversing the upper part ot the still at all. At such times, the valve 5T may be partly closed to maintain an appropriate pressure gradient in the lower run. Meanwhile the upper part ot the still is iillcd with pure nitrogen trom a stored supply, and this nitrogen is circulated through the out-going system et' pipes Q4, 25, etc. to i7, compressed, and returned as the stream l5, any necessary makeup being taken trom the stored supply. The effect of continuing this process will be to cool the whole upper part oi the still to the temperature appropriate to the spare Q3, and to store in it an ac'V cuniulation ot pure or nearly pure nitrogen lf luid, without losing any considerable part ot the reserre oil pure or nearly pure nitro-- gen saved Ytroni pre Yious runs. l/Vhen the storage ot liquid nitrogen in the upper part ot the still is suilicient, the 11p-going gaseous stream `trom the lower part of the still gradually diverted from its temporary to its final ourse by opening the valves 56 and 5T and closing the valve 57, and liquid iroiii the upper part ot the still is gradually admitted to the lower part of the still by (queuing the valve 5S. The necessary readjustment ot temperatures throughout the still will Ythen take place quickly, without any material contamination oi the pure or nearly pure nitrogenleaving the space 23.

rPhe apparatus repress. ted diagrammatically in Fig. 4 differs from that shown 'n lfig. 3 also in other respects. The stream ot nitrogen product coming from the top ot the still and passing into the pipe 24 and through the coils Q5 is withdrawn from the still shell approximately at the L. P. air.` inlet level, through the pipe 53 and is caused to cool the L. P. air from the temperature at which it leaves the interehanger, substantially to that appropriate to the L. P. air inlet level, in a small. auxiliary interchanger comprising passages 52 and 54 for the nitrogen and the L. P. air respectively. The ar rangement of Fig. 3 is preferred, but I have chosen to draw Fig. 4 in this way chiefly to avoid an unnecessarily complicated diagram.

Furthermore the liquid nitrogen cooling coils 4? in the upper runs of the train are here represented as being connected in series rather than in parallel and as being ted from the main liquid nitrogen riser 38 to which they are connected by the pipe 38 and as discharging directly into the upper part ol the train.

Furthermore in Fig. 4 such additional nitrogen liquid as may be required for refrigeration or liquid-level maintenance in the upper part of the main train, is represented as being trottled, through the valve 50 directly into the l. P. nitrogen exhaust pipe :20' trom the engine 20, and as passing through the coils 83 and 86 with the rest of the l. P. stream.

All these will be recognized as merely alternative embodiments ot the same fundamental principles embodied in the apparatus shown in Fig. 3.

As clearly indicated, the rectification train and the shell containing it are divided into two separate units at the level ot the L. P. air inlet. Thermal relation is substantially eliminated and may exist in so tar only as the necessary communications therebetween make an absolute thermal separation impossible. liy suitable lagging, however, such incidental heat exchange may be reduced to a negligible quantity. During the cooling down process described above, the H. P. air is tlirottled into the upper part of the lower unit through the valve and enters the train together with the L. P. air cooled in the auxiliary cooler. After running conditions are established the H. P. air is passed by pipe 60 into and through coils 40 and` a'tter being cooled in the lower run is fed through pipe 4l and throttle valve 42 into a region ot the upper unit to which its composition is appropriate. Under running conditions, gaseous communication between llO the lower unit and the tu per unit takes place through the pipe 56', and lio*i id com* municatiion takes place through the pipe 52%A The apparatus represented diagrammatically in Fig. 5 is a column of the familiar Claude type so inodiiied as to incorporate the more important features of my invention.

The trays 6l of the lower run of the main rectification train are in thermal contact with backward return pipes 62 or the nitrogen condensing` pipes 63 or both. The liquid from the liqueier is kept separate from that collected in the receiver (ill at the bottom of the column and is fed into the column through the pipe 65 and the valve 66 at a higher level than that at which the liquid from the receiver Get enters the train through the pipe 67 and the valve 67. The nitrogen product is withdrawn from the top of the train through the pipe 68 and the coils 69 so that its heat absorbing power may be used in the upper runs of the train.

llnliquefied and substantially uncompressed air is cooled rst in the main interchanger (not shown) and then in the auxiliary interchanger 70 and fed into the main train through the pipe 7l at a level below that at which the liquid from the receiver 6l enters the train. rlhe construction. shown in Fig. 3, in which the two passages of the interchanger l0 are incorporated in the lower part ot the column, is perhaps preferable to that shown,y and could be used in connection with this disposition of apparatus.

lt is evident that in this modification of a Claude column the backward return portion of the apparatus should be so designed and run as to give, in the receiver 64A-, not a liep uid containing` as little nitrogen as possible, but one intermediate in composition between the 2l to 25% oxygen liquid discharged from the valve G6 and the l? to oxygen liquid appropriate to the level of the L. l). air inlet il so that the four streams entering' the main rectiiication train at and above the level 7l may be as well distributed as possible.

The various possibilities for subcoolinn` the three liquid streams entering;` the upper part of the main rectification train that have been described above, not illustrated iu the drawing` to avoid undue complexity, but l wish to have it understood that lY consider this aspect as within the scope of my invention.

ln the following` formulation of the elements of my invention7 .ll wish certain er:A pressions, which are used as convenient ab breviations for much longer expressions7 to be understood in each and every case to have the meanings assigned to them in this and the following` paragraphs. The word rec tilication shall be understood to refer to any operation wherein a stream of a gaseous mixture, moving' from a warmer to a colder region, and a st earn of liquid, mow ingy in the opposite direction, are brought into intimate physical well as thermal Contact with each other, so that either .ma-- teria-l or heat can pa s freely from on;` stream to vthe other. am well aware that the word is used by some authorities in a more restricted sense; desirev however, that in this specification and the accompany-- ing claims, it be understood to refer to any operation meeting the above very general deiinition, including operations in which either material or heat or both is added to or withdrawn from either or both streams in other ways than by transference between the two streams.

lt is further desired that the words train and rectification train be unders refer either to a space which is intended te contain two interacting` streams l a y etc. intended to conline and control the said streams, as the context may indicate; but not to any space intended to contain any other stream or material, or to any pipe, passage, chambery or other strut `tural element that may be contiguous to a rectilicatien train, er contained within the saine structure or column that contains a reetilication train. ln p; 'ticular, it is de sired that the words train7 and rectilicw tion train shall not be understood to inn clude a boiling-off pool, such as is o n placed immediately below the bottom of a train in the known art, or a condenser, sucii as is ofte i placed immediately above the top of a train in the known art, unless it shall appear that rectilication is intended to take place 'in one or the other of these structures to an extent materially greater than that necessarily .incidental to their prii'nary fund tions as iinitiators of the `gaseous and liquid streams in tue train itself.

Reference will be made in the accompany# ingr claims to passino or introducing material or a stream oli duid into a train. Tl' is desired that neither the liquid reflux baffles trays All from a condenser immediately above the lop of a train nor the gaseous efflux from a boil- '-olf pool immediately below the bottom .i a train shall be regarded as one of the .streams explicitly mentioned in the append ed clams as passed or introduced into the train ln other words, l wish it understood that the streams referred to .in the claims are streams different from either ot those just mentioned.

lt is desired that the letters L. P.. when applied lo a tluid or 'to a train, shall mean that the pressure of the fluid er that of the contents of the train is in the usual case substantially the same as the pressure of the atmosphere, or greater than atmospheric pressure only by such an amount as might loo reasonably be provided to secure adequate flow through the various parts ot the separation, and to permit appropriate control ot the said flow. if, however, it be desired to run the whole separation apparatus with a discharge pressure greater than atmospheric so that the usei'ul product may be passed directly to some other apparatus not a part oi the separation apparatus or et the power recovery apparatus primarily associated therewith, then it is desired that the letters L. P. be understood to rei'er to a pressure equal to or only slightly greater than. the desired discharge pressure, as eX lained above. It is desired that the letters P., when similarly used, be understood to define the pressure as materially greater than that dened above as L. P., but less than the critical pressure o1 the fluid in question. It is desired that the letters ll. P., when similarly used, be understood to define the pressure as materially greater than the critical pressure of the liuid in question.

rlhis application is a division of my copending application Serial No. 394,544, filed lluly 7, 1920.

I Claim:

l. The combination with an L. P. rectification train oit' means for introducing material at a `plurality oi' levels and means for :distracting heat irom the contents of the train at levels just .above each level other than at the top at which material is introduced into the train.

2. The combination with a. rectification train of means for abstracting heat 'from the contents of the train between the top of the train and the lowest level at which material is introduced, said means terminating its effective thermal contact with the contents ot the train above or substantially at said level and means tor imparting heat to the contents ot the train between the bottom ot 'the train and the lowest level at which material is introduced into the train, said means termii'iating` its eifective thermal contact with the contents of the train below or substantially at said level.

il. The combination with a rectification train et means for abstracting` eat from the contents ot the train at a plurality of ditlcrent .levels between the top oi the train and the lowest level at which material is introduced, said means terminating its effecf tive thermal contact with the contents of the train above or substantially at said level and means for iinliartingr heat to the contents of the train at a plurality o t' different levels between the bottom oit the train and the lowest level at which material is introduced into the train, said means terminating its effective thermal. contact with the contents oi"A the train below or substantially at said level.

combination with a rectilication 'tor abstracting heat from f ts of the train. at substantially all is oetireen the top of the train and the lowest le el at which material is introduced, means terminating its effective thermal contact with the contents of the train above or 'substantially at said level and means for impartingj heat to the contents of the train at substantially all levels between the bottom olf the train and the lowest level at which inaterial is introduced into the train, said means terminating its effective thermal contact with the contents of the train below enti ally at said level.

o. lee combination with a rectification train, of means tor cooling an L. P. stream of a gaseous mixture, means for introducine; the cooled gaseous stream into the train intermediate the ends thereof, and means ttor bringing material in thermal but not physical contact with the contents oit the ti...n iter abstracting heat` from said contents between the top or the train and the level at which the gaseous stream enters, said. last named means terminating substantially at said level.

6. The combination with 'a rectification lrain, ot means for cooling` an L. P. stream or a Oaseous mixture, means for introducing the coole-fl r-.s stream into the train ind' ite the ends thereof, and means for iri l .material in thermal but not physin -al contact with the contents of the train tor abstra ting heat from said contents at a plurality oi different levels between the top oil the train and the level at which the gaseous stream. enters the train, said last named means terminating substantially at f. d level.

M. The combination with a rectification train. oit means for cooling an L. P. stream a gaseous mixture, means for introducing he cooled gaseous stream into the train infliate the ends thereof. and means for i ging material in thermal but not physil Contact with the contents of the train Y abstracting heat from said contents at .illy all levels between the top and evel at which the gaseous stream enters, last named means terminating substantially said level. l

t' combination with a rectification ol' means for coolingi an L. P. stream 'afcous mixture, means ttor introducing i. cooled gaseous stream into the train intermediate the ence thereof, means for bring material in thermal but not physical contact with the contents o'f the train nermediate. the top thereoi' and the level at ich the gaseous stream enters and termi- .l in@v substantially at said level, for abstracting heat from said contents, and means :tor bringing material in thermal but not physical contact with the contents of i im 'intermediate Atlie bot c n il ich tlie C,

i stream educ,l

intermee wringing eli lie iop the gaseous lor .qnseoiiis substa ointeziitsi une i plivsieel concentents ot the trein et subell levels between the, bottom no the level :it which the gaseous s i bstentislly ters Luid tcrininetine he combination with rectification trai of means :t'o-r introducino stream of e gaseous mixture into intermediate its ends, seid ineens 1 Ineens terminating substantially n" ot' the point of entrance ol' ssi-rl passing tbe stream upwardly it, not physical contact with tlie contents oit t trein et a, plurelity ot successively lii levels between. the bottoni thereof sind said point of entrance.

l2. The combination with n. rectification trein, of means for introducing* :in li. T. stream oit e gaseous inzit/ure into the. trein intermediate its ends` seid means including means terminating` substantially at the level the point of entrance ot seid str/uni i( passing the stream upwsidly in thermal li not physical Contact with the contents l trein at substruntizillj7 all levels between bottom thereof and the seid point ot trance.

:LA7-lese low u? elle tr tiozini, und ineens 1 eem in tbernnil with ilie eoutcnls ot levels li etween tlien l but not en 'lie '.'l el;

introduced on with forming i s l eens for pi stream in llicillnel .but not i y,

tue tren with the contents or of levels between tlf.- bottom et tbe und the, lowest level et wliicli introduced into the train, nieuns ing); the ijn'essure oit the seid sti'eifimeans for tlien putting; the seid sireA l but not physical coiitm-t w therme..

L e.. nluieliti.' oA

contents of t U tlie 'rein und Lil;

is introt1 uced .S stieum enters for ebstructinov liest rom the seid contentsj seid lest named ineens tei'- niineting substuntiall)7 nt seid level.

18. A rectiicetion trein composed oll two thern'iell3 seperated sections7 ineens lfoi l ssin liquid 'from the bottom ot tue upper 0 section into tlie top of the lower Leetion. and ineens loi' passing gas troni the top of: the lower section into tlie bottom or the upper section.

19. A. rectification trein composed of u series of thermally separated sectionsa means for passing liquid from the bottom ot each but the lowest section into tbe top of the next lower section7 ond menus for passing gas from the top of each but the highest fl l' ou lill) section into the bottom of the next higher section.

20. Apparatus means for passing` the train at e level thermal division.

2l. Apparatus as deliiietl in claim 19 aiirl means for passing ot lluitl into the train at levels adjacent to the different levels of thermal sepermion. respectively.

22. Apparatus defined iii 'claim 2l and means for bringing cold products from the top of the coldest section of the series immediately into thermal lout riot physical coritact With the contents of the train et a level or levels near the bottom or' one or more ol' the sections.

cleliiietl iri a stream aflj aceA t il S 23. Apparatus es defined in claim 21 and means for imparting heat to the contents of the train at a level or levels near the top of one or more of the sections.

24. Apparatus -as deliietl in claim 2l, said y last mentioned means including means for putting one or more or the said streams o -luicl under a pressure higher than that in the tra-in, means for putting' said streams into thermal but not physical Contact with the contents oit the upper part of each or one or more or' the sections of the trein, and means for reducing the pressure of tlie said streams before they enter the trein.

In testimony whereof l aflix my signature.

HARVEY N. DAVS. 

